Since the release of Bedrock 1.14 yesterday I have seen The Drowned spawn repeatedly on bottom half slabs and glass where they never spawned before, that's why I use those blocks in mass. PLEASE don't tag this as a duplicate! This is a Current issue with 1.14. the video shows them appearing right after I reset by going to peaceful.

I have also now had zombies do the same in a previously secure area. hopefully this isn't happening with creepers, that could mess up some things, as they usually do.